I used Adobe Illustrator to create a line drawing of something I wanted to include in a Doodly video. I used a combination of 1, 2 and 3 point lines in the illustration. I imported it, and editing the drawing pattern. The problem is that when Doodly is drawing the image, it looks very faint, and the 1 and 2 point lines are not visible at all, until the draw animation is complete, and then it looks normal. I tried with both a transparent background, and a white background, and both JPG and PNG export modes. I did not try an SVG, but could, if that's the fix.
I had the same problem with a black and white JPEG, the lines on my import were no thicker than some of the ones in the stock material, but when they are being drawn the full lines appear immediately.